The present invention is related to processes for making strong, soft, absorbent fibrous webs. More particularly, the present invention is concerned with dewatering of fibrous webs.
Fibrous structures, such as paper webs, are produced by a variety of processes. For example, paper webs may be produced according to commonly-assigned U.S. Pat. No. 5,556,509, issued Sep. 17, 1996 to Trokhan et al.; U.S. Pat. No. 5,580,423, issued Dec. 3, 1996 to Ampulski et al.; U.S. Pat. No. 5,609,725, issued Mar. 11, 1997 to Phan; U.S. Pat. No. 5,629,052, issued May 13, 1997 to Trokhan et al.; U.S. Pat. No. 5,637,194, issued Jun. 10, 1997 to Ampulski et al.; and U.S. Pat. No. 5,674,663, issued Oct. 7, 1997 to McFarland et al., the disclosures of which are incorporated herein by reference. Paper webs may also be made using through-air drying processes as described in commonly-assigned U.S. Pat. No. 4,514,345, issued Apr. 30, 1985 to Johnson et al.; U.S. Pat. No. 4,528,239, issued to Trokhan, Jul. 9, 1985; U.S. Pat. No. 4,529,480, issued Jul. 16, 1985 to Trokhan; U.S. Pat. No. 4,637,859, issued Jan. 20, 1987 to Trokhan; and U.S. Pat. No. 5,334,289, issued Aug. 2, 1994 to Trokhan et al. The disclosures of the foregoing patents are incorporated herein by reference.
Removal of water from the paper in the course of paper-making processes typically involves several steps. Initially, an aqueous dispersion of fibers typically contains more than 90% water and less than 10% papermaking fibers. Almost 99% of this water is removed mechanically, yielding a fiber-consistency of about 20%. Then, pressing and/or thermal operations, and/or through-air-drying, or any combination thereof, typically remove less than about 1% of the water, increasing the fiber-consistency of the web to about 60%. Finally, the remaining water is removed in the final drying operation (typically using a drying cylinder), thereby increasing the fiber-consistency of the web to about 95%.
Because of such a great amount of water needed to be removed, water removal is one of the most energy-intensive unit operations in industrial paper-making processes. According to one study, paper-making is the leading industry in total energy consumption for drying, using more than 3.75xc3x971014 BTU in 1985 (Salama et al., Competitive Position Of Natural Gas: Industrial Solids Drying, Energy and Environmental Analysis, Inc., 1987). Therefore, more efficient methods of water removal in the paper-making processes may provide significant benefits for the paper-making industry, such as increased machine capacity and reduced operational costs.
It is known in the papermaking arts to use steady-flow impingement gas and cylinder dryers to dry a paper web. (See, for example, Polat et al., Drying Of Pulp And Paper, Handbook Of Industrial Drying, 1987, pp. 643-82). Typically, impingement hoods are used together with Yankee cylinder dryers for tissue products. In webs having relatively low basis weights of about 8-11 pounds per 3000 square feet, water is removed in about 0.5 seconds. This corresponds to an evaporation rate of about 42 pounds per hour per square feet, with about 75% of the total evaporation being performed by the impingement hood. The drying rates of paper products having relatively heavier basis weights are considerably slower. For example, newsprint, having a basis weight of about 30 pounds per 3000 square feet, has the evaporation rate of about 5 pounds per hour per square feet on the cylinder dryers. See, for example, P. Enkvist et al., The Valmet High Velocity and Temperature Yankee Hood on Tissue Machines, presented at Valmet Technology Days ""97, Jun. 12-13, 1997, at Oshkosh, Wis., USA.
It is also known to use a sonic energy, such as that generated by steam jet whistles, to facilitate removal of water from various products, including paper. U.S. Pat. No. 3,668,785, issued to Rodwin on Jun. 13, 1972, teaches sonic drying and impingement flow drying in combination for drying a paper web. U.S. Pat. No. 3,694,926, issued to Rodwin et al. on Oct. 3, 1972, teaches a paper dryer having a sonic drying section through which the web is passed and subjected to high intensity noise from grouped noise generators, to dislocate moisture from the web. U.S. Pat. No. 3,750,306, issued to Rodwin et al. on Aug. 7, 1973, teaches sonic drying of webs and rolls, involving steam jet whistles spaced along trough-like reflectors and low pressure secondary air to sweep displaced moisture clear of the traveling web.
The foregoing teachings provide a means for generating sonic/acoustic energy and a separate means for generating steady-flow impingement/wiping air. Generating the acoustic energy in accordance with the prior art by such means as noise generators, steam whistles, and the like requires very powerful acoustic sources and leads to a significant power consumption. It is well known in the art that the efficiency of the conventional noise generators, such as sirens, horns, steam whistles, and the like typically do not exceed 10-25%. An additional equipment, such as auxiliary compressors to pressurize air, and amplifiers to generate the desired sound pressure, may also be necessary to reach a desired drying effect.
Now, it has been found that impingement of a paper web with air or gas having oscillatory flow-reversing movement, as opposed to a steady-flow impingement of the prior art, may provide significant benefits, including higher drying/dewatering rates and energy savings. It is believed that an oscillatory flow-reversing impingement air or gas having relatively low frequencies is an effective means for increasing, relative to the prior art, heat and mass transfer rates in papermaking processes.
Pulse combustion technology is a known and viable commercial method of enhancing heat and mass transfer in thermal processes. Commercial applications include industrial and home heating systems, boilers, coal gassification, spray drying, and hazardous waste incineration. For example, the following U.S. Patents disclose several industrial applications of pulse combustion: U.S. Pat. No. 5,059,404, issued Oct. 22, 1991 to Mansour et al.; U.S. Pat. No. 5,133,297, issued Jul. 28, 1992 to Mansour; U.S. Pat. No. 5,197,399, issued Mar. 30, 1993 to Mansour; U.S. Pat. No. 5,205,728, issued Apr. 27, 1993 to Mansour; U.S. Pat. No. 5,211,704, issued May 18, 1993 to Mansour; U.S. Pat. No. 5,255,634, issued Oct. 26, 1993 to Mansour; U.S. Pat. No. 5,306,481, issued Apr. 26, 1994 to Mansour et al.; U.S. Pat. No. 5,353,721, issued Oct. 11, 1994 to Mansour et al.; and U.S. Pat. No. 5,366,371, issued Nov. 22, 1994 to Mansour et al., the disclosures of which patents are incorporated by reference herein for the purpose of describing pulse combustion. An article entitled xe2x80x9cPulse Combustion: Impinging Jet Heat Transfer Enhancementxe2x80x9d by P. A. Eibeck et al, and published in Combustion Science and Technology, 1993, Vol. 94, pp. 147-165, describes a method of convective heat transfer enhancement, involving the use of pulse combustor to generate a transient jet that impinges on a flat plate. The article reports enhancements in convective heat transfer of a factor of up to 2.5 compared to a steady-flow impingement.
The applicant believes that the oscillatory flow-reversing impingement can also provide significant increase in heat and mass transfer in web-dewatering and/or drying processes, relative to the prior art dewatering and/or drying processes. In particular, it is believed that the oscillatory flow-reversing impingement can provide significant benefits with respect to increasing paper machine rates, and/or reducing air flow needs for drying a web, thereby decreasing size of. the equipment and capital costs of web-drying/dewatering operations andxe2x80x94consequentlyxe2x80x94an entire papermaking process. In addition, it is believed that the oscillatory flow-reversing impingement enables one to achieve a substantially uniform drying of the differential-density webs produced by the current assignee and referred to herein above. It is now also believed that the oscillatory flow-reversing impingement may be successfully applied to dewatering and/or drying of fibrous webs, alone or in combination with other water-removing processes, such as through-air drying, steady-flow impingement drying, and drying-cylinder drying.
To be able to effectively remove water from the web, the oscillatory flow-reversing air or gas should in most cases act upon the web in a substantially uniform manner, especially across the web""s width (i.e., in a cross-machine direction). Alternatively, one might desire to differentiate, in a particular pre-determined manner, the application of the oscillatory impingement gas across the width of the web, thereby controlling relative moisture content and/or drying rates of differential regions of the web. In either instance, the control over the distribution of the oscillatory flow-reversing air or gas throughout the surface of the web, and particularly in the cross-machine-direction, may be important to the effectiveness of the process of removing water from the web.
Paper webs produced on modern days industrial-scale paper machines have width of about from 100 to 400 inches, and travel at linear velocities of up to 7 feet per minute. Such a width, coupled with a high-speed movement of the web creates certain difficulties of controlling (presumably uniform) distribution of the oscillatory gas throughout the surface of the web. Existing apparatuses for generating oscillatory flow-reversing air or gas, such as, for example, pulse combustors, are not well adapted, if at all, to generate a required substantially uniform oscillatory field of the flow-reversing air or gas across a relatively large area.
Accordingly, the present invention provides a process and an apparatus for removing water from fibrous webs, using the oscillatory flow-reversing impingement gas. The present invention also provides a water removing apparatus comprising a rotary air valve pulse generator. The present invention also provides a gas-distributing system allowing one to effectively control the distribution of the oscillatory flow-reversing air or gas throughout the surface of the web. The present invention further provides a gas-distributing system that creates a substantially uniform application of the oscillatory flow-reversing air or gas onto the web.
The present invention provides a novel process and an apparatus for removing water from a fibrous web by using oscillatory flow-reversing air or gas as an impinging medium. The apparatus and the process of the present invention may be used at various stages of the overall papermaking process, from a stage of forming an embryonic web to a stage of post-drying. Therefore, the fibrous web may have a starting moisture content in a broad range, from about 1% to about 99%, i.e., a fiber-consistency of the web may be from about 99% to about 1%.
In its process aspect, the present invention comprises the following steps: providing a fibrous web; providing an oscillatory flow-reversing impingement gas having a predetermined frequency; providing a gas-distributing system comprising at least one discharge outlet and designed to deliver the oscillatory flow-reversing impingement gas onto a predetermined portion of the web; and impinging the oscillatory flow-reversing gas onto the web through the at least one outlet, thereby removing moisture from the web.
The first step of providing a fibrous web may be preceded by steps of forming such a web, including the steps of providing a plurality of papermaking fibers. The present invention also contemplates the use of the web formed by dry-air-laid processes or the web that has been rewetted. The web may have a non-uniform moisture distribution prior to water removal by the process and the apparatus of the present invention, i.e., the fiber-consistency of some portions of the web may be different from the fiber-consistency of the other portions of the web.
A water-removing apparatus of the present invention has a machine direction and a cross-machine direction perpendicular to the machine direction. The apparatus of the present invention comprises a web support designed to receive a fibrous web thereon and to carry it in the machine direction; at least one pulse generator designed to produce oscillatory flow-reversing air or gas and comprising a rotary air valve generator having frequency from about 15 Hz to about 1,500 Hz; and at least one gas-distributing system in fluid communication with the pulse generator for delivering the oscillatory flow-reversing air or gas to a predetermined portion of the web. The gas-distributing system terminates with at least one discharge outlet juxtaposed with the web support. In one embodiment the gas distributing system comprises a blow box juxtaposed with the web support. The web support and the discharge outlet form an impingement region therebetween. The impingement region is defined by an impingement distance xe2x80x9cZ.xe2x80x9d The impingement distance Z is, in other words, a clearance between the at least one discharge outlet and the web support. The oscillatory flow-reversing gas may be impinged onto the web to provide a substantially even distribution of the gas throughout the impingement area of the web. Alternatively, the oscillatory gas may be impinged onto the web to provide an uneven distribution of the gas throughout the impingement area of the web thereby allowing control of moisture profiles of the web.
According to the present invention, the pulse generator is a device which is designed to produce oscillatory flow-reversing air or gas having a cyclical velocity/momentum component and a mean velocity/momentum component. An acoustic pressure generated by the pulse generator is converted to a cyclical movement of large amplitude, comprising negative cycles alternating with positive cycles, the positive cycles having greater momentum and cyclical velocity relative to the negative cycles.
The xe2x80x9cgas-distributing systemxe2x80x9d defines a combination of tubes, tailpipes, blow boxes, etc., designed to provide an enclosed path for the oscillatory flow-reversing air or gas produced by the pulse generator, and to deliver the oscillatory flow-reversing air or gas to a pre-determined impingement region, where the oscillatory flow-reversing air or gas is impinged onto the web, thereby removing water therefrom. The gas-distributing system is designed such as to minimize, and preferably avoid altogether, disruptive interference which may adversely affect a desired mode of operation of the pulse generator or oscillatory characteristics of the flow-reversing gas generated thereby. The gas-distributing system delivers the flow-reversing impingement air or gas onto the web through at least one discharge outlet, or nozzle. The frequency of the oscillatory flow-reversing impingement air or gas is in a range of from about 15 Hz to about 1,500 Hz, more specifically from 15 Hz to 500 Hz, still more specifically from 15 Hz to 250 Hz, depending on a type of the pulse generator and/or desired characteristics of the water-removing process.
A Helmholtz-type resonator may be used in the pulse generator of the present invention. Typically, the Helmholtz-type pulse generator may be tuned to achieve a desired sound frequency.
Various embodiments of the pulse generator include, Wthout limitation, pulse combustors, infrasonic devices, devices comprising solenoid valves, fluidic valves, rotary valves, butterfly valves, vibrating mechanical elements, rotating lobes, and pizeo electric element. One embodiment of the pulse generator comprises a rotary valve pulse generator. In the rotary valve pulse generator, temperature-controlled air is forced under pressure, through a coaxial rotating air valve to produce pressure pulses which are forced through a Helmholtz resonator. The frequency of pulses is controlled by a rotational speed of the rotary air valve. The amplitude of the pressure pulses is increased by the resonance created by the standing acoustic wave within the Helmholtz resonator. The oscillatory pressure is converted to oscillatory flow reversing flow at the discharge end of the resonance tubes and distributors of the gas-distributing system.
The oscillatory flow-reversing impingement air or gas has two components: a mean component characterized by a mean velocity and a corresponding mean momentum; and an oscillatory, or cyclical, component characterized by a cyclical velocity and a corresponding cyclical momentum. The oscillatory cycles during which the combustion gas moves xe2x80x9cforwardxe2x80x9d from the combustion chamber, and into, through, and from the gas-distributing system are positive cycles; and the oscillatory cycles during which a back-flow of the impingement gas occurs are negative cycles. An average amplitude of the positive cycles is a positive amplitude, and an average amplitude of the negative cycles is a negative amplitude. During the positive cycles, the impingement gas has a positive velocity directed in a positive direction towards the web disposed on the web support; and during the negative cycles, the impingement gas has a negative velocity directed in a negative direction. The positive direction is opposite to the negative direction, and the positive velocity is opposite to the negative velocity. The positive velocity component is greater than the negative velocity component, and the mean velocity has the positive direction.
The pulse generator produces an intense acoustic pressure. Due to the open end of the resonance tube, the acoustic pressure is reduced at the exit of the resonance tube. This drop in the acoustic pressure results in a progressive increase in cyclical velocity which reaches its maximum at the exit of the resonance tube. It may be beneficial to use the Helmholtz-type pulse generator in which the acoustic pressure is minimal at the exit of the resonance tubexe2x80x94in order to achieve a maximal cyclical velocity in the exhaust flow of oscillatory impingement gases. The decreasing acoustic pressure beneficially reduces noise typically associated with sonically enhanced processes of the prior art.
At the exit of the gas-distributing system, the cyclical velocity, ranging from about 1,000 ft/min to about 50,000 ft/min, and more specifically from about 2,500 ft/min to about 50,000 ft/min, is calculated based on the measured acoustic pressure in the combustion chamber. A more specific cyclical velocity is from about 5,000 ft/min to about 50,000 ft/min. The mean velocity is from about 1,000 ft/min to about 25,000 ft/min, more specifically from about 2,500 ft/min to about 25,000 ft/min, and still more specifically from about 5,000 ft/min to about 25,000 ft/min.
It is believed that for the web having moisture content from about 10% to about 60%, the apparatus and the process of the present invention allow one to achieve the water-removal rates up to 150 lb/ft2xc2x7hr and higher. In order to achieve the desired water-removal rates, the oscillatory flow-reversing impingement gas should preferably form an oscillatory xe2x80x9cflow fieldxe2x80x9d substantially uniformly contacting the web throughout the surface of the web. One way of accomplishing it is to cause the flow of the oscillatory gas from the gas-distributing system be substantially equally split and impinged onto the drying surface of the web through a network of the discharge outlets. In such an embodiment, the apparatus of the present invention is designed to discharge the oscillatory flow-reversing impingement air or gas onto the web according to a pre-determined, and preferably controllable, pattern. A pattern of distribution of the discharge outlets may vary. One pattern of distribution comprises a non-random staggered array.
The discharge outlets of the gas-distributing system may have a variety of shapes, including but not limited to: a round shape, generally rectangular shape, an oblong slit-like shape, etc. Each of the discharge outlets has an open area xe2x80x9cAxe2x80x9d and an equivalent diameter xe2x80x9cD.xe2x80x9d A resulting open area xe2x80x9cxcexa3Axe2x80x9d is a combined open area formed by all individual open areas of the discharge outlets together. An area of a portion of the web impinged upon by the oscillatory flow-reversing impingement field at any moment of the continuous process is the impingement area xe2x80x9cE.xe2x80x9d
In a continuous process, the web is supported by the web support traveling in the machine direction. A means for controlling the impingement distance may be provided, such as, for example, conventional manual mechanisms, as well as automated devices, for causing the outlets of the gas-distributing system and the web support to move relative to each other, thereby changing the impingement distance. Prophetically, the impingement distance may be automatically adjustable in response to a signal from a control device, measuring at least one of the parameters of the dewatering process or one of the parameters of the web. In one embodiment, the impingement distance may vary from about 0.25 inches to about 24.0 inches, and more specifically, from about 0.25 to about 12 inches. The impingement distance defines an impingement region, i.e., the region between the discharge outlet(s) and the web support. In the preferred embodiment, a ratio of the impingement distance Z to the equivalent diameter D of the discharge outlet (i.e., Z/D) is from about 1.0 to about 10.0. A ratio of the resulting open area xcexa3A to the impingement area E (i. e., xcexa3A/E) is from 0.002 to 1.000, more specifically from 0.005 to 0.200, and still more specifically from 0.010 to 0.100.
In one embodiment, the gas-distributing system comprises at least one blow box. The blow box comprises a bottom plate which may have a plurality of discharge outlets therethrough. Alternatively, the bottom plate may have a slot-like discharge opening extending in the cross-machine direction, i.e., across the width of the web being dried or dewatered. The blow box may have a substantially planar bottom plate. Alternatively, the bottom plate of the blow box may have a non-planar or curved shape, such as, for example, a convex shape, or a concave shape. In one embodiment of the blow box, a generally convex bottom plate is formed by a plurality of sections.
An angled application of the oscillating flow-reversing air-or gas may be beneficially used in the present invention. Angles formed between the general surface of the web support and the positive directions of the oscillating streams of air or gas through the discharge outlet may range from almost 0 degree to 90 degrees. These angles may be oriented in the machine direction, in the cross machine direction, and in the direction intermediate the machine direction and the cross-machine direction.
A plurality of the gas distributing systems can be used across the width of the web. This arrangement allows a greater flexibility in controlling the conditions of the web-dewatering process across the width of the web. For example, such arrangement allows one to control the impingement distance individually for differential cross-machine directional portions of the web. If desired, the individual gas-distributing systems may be distributed throughout the surface of the web in a non-random, for example, staggered-array, pattern.
The oscillatory field of the flow-reversing impingement gas may beneficially be used in combination with a steady-flow (non-oscillatory) impingement gas impinged onto the web. One preferred embodiment comprises sequentially-alternating application of the oscillatory flow-reversing gas and the steady-flow gas. One of or both the oscillatory gas and the steady-flow gas can comprise jet streams having the angled position relative to the web support.
The web support may include a variety of structures, for example, papermaking band or belt, wire or screen, a drying cylinder, etc. In one embodiment, the web support travels in the machine direction at a velocity of from 100 feet per minute to 10,000 feet per minute. More specifically, the velocity of the web support is from 1,000 feet per minute to 10,000 feet per minute.
The apparatus of the present invention may be applied in several principal steps of the overall papermaking process, such as, for example, forming, wet transfer, pre-drying, drying cylinder (such as Yankee) drying, and post-drying. One location of the impingement region is an area formed between a drying cylinder and a drying hood juxtaposed with the drying cylinder, in which instance the web support may comprise a surface of the drying cylinder. In one embodiment, the impingement hood is located on the xe2x80x9cwet endxe2x80x9d of the cylinder dryer. The drying residence time can be controlled by the combination of hood wrap around the drying cylinder and machine speed. The process is particularly useful in the elimination of moisture gradients present in the differential-density structured paper webs.
One embodiment of the web support comprises a fluid-permeable endless belt or band having a web-contacting surface and a backside surface opposite to the web-contacting surface. This type of the web support comprises, in one embodiment, a framework joined to a reinforcing structure, and at least one fluid-permeable deflection conduit extending between the web-contacting surface and the backside surface. The framework may comprise a substantially continuous structure. Alternatively or additionally, the framework may comprise a plurality of discrete protuberances. If the web-contacting surface is formed by a substantially continuous framework, the web-contacting surface comprises a substantially continuous network; and the at least one deflection conduit comprises a plurality of discrete conduits extending through the substantially continuous framework, each discrete conduit being encompassed by the framework.
Using the process and the apparatus of the present invention one can simultaneously remove moisture from differential-density structured webs. The dewatering characteristics of the oscillatory flow-reversing process is dependent to a significantly lesser degree, if at all, upon the differences in density of the web being dewatered, in comparison with the prior art""s conventional processes using a drying cylinder or through-air-drying processes. Therefore, the process of the present invention effectively decouples the water-removal characteristics of the dewatering processxe2x80x94most importantly water-removal ratesxe2x80x94from the differences in the relative densities of the differential portions of the web being dewatered.
The process of the present invention, either alone or in combination with the through-air-drying, can eliminate the application of the drying cylinder as a step in the papermaking process. One of the preferred applications of the process of the present invention is in combination with through-air-drying, including application of pressure generated by, for example, a vacuum source. The apparatus of the present invention can be beneficially used in combination with a vacuum apparatus, such as, for example, a vacuum pick-up shoe or a vacuum box, in which instance the web support can be fluid-permeable. The vacuum apparatus is preferably juxtaposed with the backside surface of the web support, and more preferably in the area corresponding to the impingement region. The vacuum apparatus applies a pressure to the web through the fluid-permeable web support. In this instance, the oscillatory flow-reversing gas created by the pulse generator and the pressure created by the vacuum apparatus can beneficially work in cooperation, thereby significantly increasing the efficiency of the combined dewatering process, relative to each of those individual processes.
Optionally, the apparatus of the present invention may have an auxiliary means for removing moisture from the impingement region, including the boundary layer. Such an auxiliary means may comprise a plurality of slots in fluid communication with an outside area having the atmospheric pressure. Alternatively or additionally, the auxiliary means may comprise a vacuum source, and at least one vacuum slot extending from the impingement region and/or an area adjacent to the impingement region to the vacuum source, thereby providing fluid communication therebetween.
The present invention is believed to provide high water-removal rates and low air flow requirements, that results in reduced capital costs. The present invention is also believed to enable the fibrous web to tolerate high temperatures due to pulsating flows and ensure a reduced thermal damage to the fibrous web being dewatered or dried.